In the manufacture of electronic devices, printed circuit boards are widely used both to support discrete electronic components and to provide the circuitry between the components. The printed circuit boards which are most commonly employed have a central core which is typically a dielectric material which has applied on its surface a layer of a conductive material such as a copper or silver. The conductive material may be applied to one or both of the surfaces of the core. The layer or layers of the conductive material are etched or otherwise processed to provide circuits of a predetermined geometrical configuration. Printed circuit boards are used when a large number of substantially identical devices are to be produced with a given circuit. It is important that the printed circuits be accurately reproduced from one board to another in the precise predetermined design configuration in order to obtain the required electrical performance from the fully assembled electronic devices.
In the manufacture of printed circuit boards difficulties are often encountered in accurately reproducing the required circuits on the circuit boards within the design parameters. A common problem which is encountered is that excessive amounts of the conductive material is removed or the conductive material is cracked or otherwise deformed to cause an "open" in the circuit. Another common problem which is encountered is that a portion of the conductive material which is supposed to be removed remains and a "short" develops in the circuitry. A still further problem is that often tramp conductive materials short the circuits causing defects in the electrical characteristics of the board.
One of the most commonly used methods to commercially check the quality of the circuits is to optically examine each of the circuits under relatively low magnification, for example 10X magnification, to determine if there are any deviations from the design pattern of the printed circuit. This method has proven to be relatively unsatisfactory as the quality control inspector must visually examine each of the circuits in comparison to a standard. Because of human error, numerous defective circuits are indicated to be satisfactory. A part of the problem is a result of some of the defects being relatively small and unnoticeable even under magnification. In addition, there a human fatigue factor is encountered in examining a large number of similar circuits in comparison to a standard.
The circuitry of the printed circuit boards can be electrically tested by determining the continuity or discontinuity of the circuits by providing electrical connections to each of the often hundreds of individual contact areas in a given circuit to test the continuity of the circuit. This type of electrical testing is, however, very expensive and time consuming.
It would be highly desirable if a method could be provided to improve the visual inspection of printed circuit boards to provide the accuracy and other desirable features of electrical testing.